Today's tough is isomorphic to 09/20's.<br>
Proof : <br>
1) Turn today's grid 90° counterclockwise.<br>
2) Exchange columns 1 and 2, exchange columns 9 and 8 .<br>
3) Exchange columns 2 and 3, exchange columns 8 an 7.<br>
4) Substitute numbers this way : 1->7->5->4->9->8->2->3->1.<br>
What do you think you get now? Exactly 09/20's tough grid.<br>

Now please note that operations 1,2,3,4 transform a correct solution of today's into a correct solution of 09/20's. Furthermore, operations 1,2,3,4 can be backed (beware the order) to transform a correct solution of 09/20's into a correct solution of today's.<br><br>

Think a bit and you'll realize that there's no point in proving again uniqueness of today's, as it suffices to transform 09/20's proof (find it in "proofs" link) by operations 4backed, 3backed, 2backed, 1backed to obtain today's proof of uniqueness.<br><br>

My apologies to mathematician readers, as all this is clearly evident to them and needs not developing.<br><br>

My apologies also to non-mathematician readers, as all this must seem a little theoretic to them and needing more explanations.<br><br>

Eventually, for what reader exactly did I post all this?